Ning's business model is based on community marketing so even in a private model (where a group creator only wants selected members to participate) anyone invited can then invite others.
There is an option for the group creator to approve all new member requests but that's a little anti-social when your private group of 'best friends' doesn't get the model and invites 5 of her friends to join also. http://networkcreators.ning.com/forum/topic/show?id=492224%3ATopic%3A129657
The creators group on Ning addresses this issue with a CSS hack to hide the "invite" privilege for non-admin memebrs but the share feature still allows for an invite to be sent out. (I have not seen this share email but it's mentioned in the post)
The social networking features are really impressive but social group controls aer critical if I plan to create social networks for both my public and private groups and know that I'll be able to control the whole thing once it's actually working. The pricing for "above free" levels of bandwidth and space would seem to be enough justification for giving group creators this additional control and making this the ultimate social networking tool.